CdTe has been recognized as an attractive photovoltaic (PV) material because of its direct band gap Eg around 1.5 eV, which is near the optimal value for single p-n PV devices based on the Shockley-Queisser model. Because of its high absorption coefficient in excess of 104 cm−1 and its direct band-gap, a CdTe film about 1 μm thick is enough for absorption of ˜90% of photons with energy higher than the band gap of CdTe. However, the most efficient solar cells reported use much thicker CdTe layers (e.g., 5-10 μm). Unfortunately, at a large CdTe thickness, the availability of Te may be a concern for production levels above 20 GW. Thin film CdS/CdTe PVs have been fabricated using magnetron sputtering. (S. Marsillac, V. Y. Parikh and A. D. Compann, “Ultra-thin bifacial CdTe solar cell,” Sol. Energy Mater. Sol. Cells 91 (15-16), 1398-1402 (2007).) The use of energetic electrons, atoms, and/or ions in magnetron sputtering subsidizes the required kinetic energy for the adatom mobility at the growth interface, which is typically provided thermally with high substrate temperatures in other popular methods applied for CdTe thick-film PVs, such as closed-space sublimation (CSS) and thermal vapor transport. This results in significantly reduced processing temperatures. However, other methods for making efficient, high quality, cost-effective thin-film CdTe-based PVs at low temperatures are needed.
First generation photovoltaics (PVs) employ single p-n junctions of photoactive semiconductors with band gaps preferably around 1.5 eV to approach an upper limit of power conversion efficiency of 31% (the so-called Shockley-Queisser limit predicted theoretically for these 1st generation PVs). Although high efficiencies close to this limit have been demonstrated experimentally on lab scale PVs of other types of semiconductors, the best efficiency reported on CdTe-based PVs is about 17%. Achieving high efficiencies on commercial PV modules remains a challenge. Similarly, reducing the cost of solar energy remains a challenge.
To enhance the PV efficiency beyond the classical Shockley-Queisser limit, various 3rd generation PV ideas have been proposed—aimed primarily at improving light absorption, in particular the infrared light in the solar spectrum. One idea of tandem PVs using multiple band gaps has been successful in conversion of solar energy of broader spectrum and hence providing higher performance. Nevertheless, the associated high fabrication cost in molecular beam epitaxy (MBE) typically used for fabrication of the tandem PVs has limited their applications primarily to concentrated solar systems, in which the required bulky optics and active tracking add additional cost, hampering their large-scale commercialization. Another idea is to introduce an intermediate band (IB) inside the semiconductor band gap to explore possibility of exceeding the Shockley-Queisser limit (Luque, A., Marti, A., “Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels.” Phys. Rev. Lett. 1997, 78 (26), 5014-5017). Experimentally, IB is typically generated via impurity doping in the original photoactive semiconductors and the lack of control in the microstructure of the doped semiconductors typically leads to charge-trapping defects, resulting in charge recombination and low efficiency in IB-related PVs. The idea of using multiple quantum well (MQW) or superlattice structures adopts a narrower band gap semiconductor to form a superlattice with the original photoactive semiconductors, typically resulting in both a conduction band offset and a valence band offset. (Wu, X., Keane, J. C., Dhere, R. G., In Proceedings of the 17th European Photovoltaic Solar Energy Conference, p. 995, 2001.) The inclusion of the narrow band gap material is meant to provide light absorption in the longer wavelength spectrum, which is prohibited in the original single p-n junction PVs. However, high power conversion efficiencies beyond the classical Shockley-Queisser limit has not been obtained on these MQW and superlattice PVs. Moreover, the focus for 3rd generation MQW and superlattice PVs has been on group III-V semiconductors, such as the AlAs/GaAs system, which typically require MBE. Besides high fabrication costs, scaling up for roll-to-roll fabrication of PVs remains challenging using MBE.